1. Field of the Invention
The present invention relates to a porous optical fiber having air holes and a method for manufacturing the same.
2. Description of the Related Art
A single mode optical fiber core is made by adding Germanium or Phosphorus to glass. As shown in FIG. 1, the porous optical fiber is made of a transparent material 1, such as fused silica glass, and uniformly spaced air holes 2 are provided through out the transparent material 1 in a longitudinal direction so that the air holes 2 are arranged in parallel with an axis of the fiber.
In a porous optical fiber, a photon transition layer is made using the difference in dielectric constants between an air layer and a silica glass layer. In the same manner as an electronic band gap in a semiconductor, such a photon transition layer has a photonic stop band against a designated wavelength or optical traveling direction. That is, only light satisfying the requirements of the photon transition layer passes through the photon transition layer. In other words, the light traveling in the porous optical fiber is achieved by a Photonic Band-gap Effect and an Effective Index Effect. This is disclosed in detail by T. A. Birks et al., Electronic Letters, Vol. 31(22) p. 1941 (October 1995) and J. C. Knight et al., Proceeding of OFC, PD 3-1 (February, 1996).
The porous optical fiber has many important technical advantages. For example, the porous optical fiber supports a single mode throughout a broad wavelength and has a large mode region, thus enabling the transmission of a signal having a high optical power and a high phase separation at a telecommunication wavelength of 1.55 μm. Further, the porous optical fiber increases/decreases the nonlinearity and thus constitutes a polarized light-regulating device. Accordingly, it is expected that the porous optical fiber having the above-described characteristics will be widely applied to optical communication fields.
In the conventional techniques of manufacturing a preform for the porous optical fiber, capillary glass tubes and glass rods are stacked and bound into a bundle having a desired shape to produce a preform. However, the conventional techniques require workers to assemble the elements into the perform manually which tend to generate contaminants during the manufacturing process and require a repetitive washing step.
Since capillary glass tubes and the glass rods are stacked and bound into the bundle, the arrangement of air holes in a hexagonal shape in the preform is simple. However, when the porous optical fiber having a porous structure is drawn from the preform, since tubular members disposed at the outside area of the preform melt faster than the tubular members disposed at the inside area of the preform due to a difference of heat conductivity between the inside and outside areas of the preform, air holes disposed at the outside area of the preform are remarkably reduced or clogged as opposed to air holes disposed at the inside area of the preform. As a result, the comparatively large-sized inside air holes tend to deform into oval shapes. This deformation of air holes, generated when the porous optical fiber is drawn from the preform, causes many difficulties in continuously manufacturing a large amount of the porous optical fiber continuously.